Inter-individual differences in the activity of CYP3A4 in the small intestine contribute to the low and variable oral bioavailability observed for many drugs that are CYP3A substrates. This variability appears to be the result of large differences in the specific content of CYP3A4 in duodenal enterocytes. From a therapeutic perspective, large differences in first-pass intestinal extraction efficiency can lead to variable systemic drug exposure and variable pharmacological effects following oral administration of doses that are appropriate for the average patient, increasing the risk of therapeutic failure and adverse toxicity. The cause of variable intestinal CYP3A expression is largely unknown, but thought to involve both genetic and environmental factors. Importantly, we have shown previously that the most biologically active form of vitamin D, 1,25-dihydroxy vitamin D3 (1,25(OH)2D3), enhances transcription of the CYP3A4 gene in a VDR-dependent manner and that CYP3A4 in turn can catalyze the metabolic clearance of 1,25(OH)2D3. The overall objectives of this grant proposal are to determine whether or not intestinal CYP3A4 is regulated in vivo by the following sequential process: formation and biliary excretion of a 25(OH)D3-glucuronide conjugate, hydrolysis of the conjugate to 25(OH)D3 in the proximal intestinal lumen and its absorption into the primary enterocytes, where it is converted to 1,25(OH)2D3 by CYP27B1. Active hormone produced in the enterocyte in this manner can regulate the expression of CYP3A4 and other VDR target genes, including the calcium transport proteins TRPV6 and calbindin D9k. We will test this hypothesis by identifying and characterizing the hepatic transporters involved in the biliary excretion of vitamin D conjugates in humans and testing whether or not these conjugates can affect the expression and function of VDR target genes in cultured human enterocytes. Because CYP3A4 can catalyze the oxidative metabolism of 1,25(OH)2D3, we also propose that activation of hPXR in the small intestine by known receptor agonists enhances intestinal 1,25(OH)2D3 clearance, resulting in a decrease in the formation of calcium transporters, and a potential change in systemic indices of calcium homeostasis. We will test this hypothesis with the use of cultured human hepatocytes, enterocytes and primary tubular epithelial cells, a novel microfluidic, 3-dimensional model of the human intestinal mucosa and the conduct of an in vivo CYP3A4 interaction study in healthy volunteers. Elucidating the molecular basis of inter-individual differences in CYP3A-dependent drug metabolism could enhance the ability of the drug industry to develop safe and efficacious drugs through a clearer understanding of how other medications, the environment, and disease states might impinge on the disposition of new drug candidates that are intestinal CYP3A substrates. In addition, if our hypothesis about the participation of CYP3A4 in negative feedback control of 1,25(OH)2D3 genomic effects within the small intestine proves correct, it could point to relatively simple ways (e.g., grapefruit juice consumption) to prevent the adverse effects of potent hPXR agonists on bone health in at-risk patients.